Transparent crystalline glass



United States Patent 3 Claims. 61. 106--39) This application is a continuation-in-part of my copending United States patent application Serial No. 185,839 filed April 9, 19-62, and now abandoned.

This invention relates to transparent crystalline glass which has been prepared using ZrO as the crystal nuclei forming agent.

In Li OAl O -SiO system glass with TiO (2- 20%) added as the crystal nuclei forming a-gent, the formation of crystals in the inner portion thereof can be controlled by heat treatment. When glass of this kind is heat treated, it becomes opaque or transparent and the properties such as the coefiicient of thermal expansion and others are completely different from those of the starting material, as is already known. When system glass is prepared with TiO (220%) as the crystal nuclei forming agent, the opaque product thus obtained is slightly yellowish but is not suflicientiy colored as to be unsuitable for commercial purposes. When the product becomes transparent, the product becomes brown and the coloring is so noticeable that the resulting article is unsuitable for many commercial purposes.

Besides the above-mentioned process, processes for preparing crystalline glass by modifying system glass with P 0 (5-22%) or ZrO (510%) as the crystal nuclei forming agent are also known. In these systems, the coloring of the product is considerably reduced, but it is impossible to obtain transparent crystalline glasses. Furthermore the defoaming is very poor and crystalline glass of sufiiciently small thermal expansion cannot be obtained with these compositions.

The present invention provides transparent crystalline glass which is obtained by melting Li OAl O SiO system glass and adding ZrO (25%) as the crystal nuclei forming agent at a temperature a little below the optimum fining temperature. The product thus obtained is almost colorless and possesses a small coefficient of thermal expansion at both low and high temperatures.

In general, when glass containing ZrO is melted at a temperature a little below the optimum fining temperature, even the foams and striae are reduced but there is a tendency that numerous particles of very small size will be produced. The present invention utilizes this tendency.

The characteristic of crystalline glass of the present invention is that the permissible amount of TiO which is the cause of coloring is very small or none so the coloring of crystalline glass obtained after the heat treatment is very little.

In general, the coefficient of thermal expansion of the product of the present invention is considerably smaller Patented Mar. 22, 1966 than that of products in which Ti0 is the crystal nuclei forming agent.

For instance the coefficient of thermal expansion after thermal treatment is 9 l0 cm./cm. C. for a glass having the composition of SiO =67.l%, TiO =5.9%, LiO =4.3%, Al O =20.7%, K O=1.0%, As O =0.5% and Sb O =O.5%.

The following glass composition is one embodiment of this invention wherein ZrO is used as the crystal nuclei forming agent and its composition is SiO =69.1%,

As O 0.5% and Sb O =*0.5%. The coefficient of thermal expansion of this glass after thermal treatment is zero or almost zero.

As above stated, the product prepared by adding Zr0 as the crystal nuclei forming agent has a smaller coefficient of thermal expansion than that of the product obtained by adding TiO as the crystal nuclei forming agent, so the former can withstand the effects of thermal impact up to a high temperature.

The glass composition of the product of the present invention is as follows (percentages are by weight):

Broad, percent Preferred,

percent ZrO, 2.0-5.0 2.0-3.0 (SiO 2+Li O+A1 O 92 Furthermore,

(I) It is preferable to add a small amount of TiO for instance, up to 1.8%, when the amount of ZrO is low.

(2) When the amount of SiO exceeds the maximum, the glasses will become infusible. When the amount of SiO is below the minimum, it becomes diflicult to prepare a transparent crystalline glass.

(3) When the amount of A1 0 exceeds the maximum, it becomes difficult to prepare a transparent crystalline glass.

(4) When the amount of A1 0 is below the minimum, the coefficient of thermal expansion of the final transparent crystalline glass will be too high.

(5) When Li O exceeds the maximum, it becomes difficult to obtain transparent crystalline glass. When the amount of Li O is below the minimum, the product becomes infusible.

(6) When the amount of ZrO exceeds the maximum, the product becomes infusible, and has a tendency to crack during the thermal treatment. When the amount of ZrO is below the minimum, the ability to form the crystal nuclei is diminished, but the addition of a small amount of TiO enables the preparation of the desired crystalline glass.

(7) When the total content of (SiO +Al O +Li O) is below 85%, the characteristic of the present invention cannot be obtained.

(8) Other additives, for instance, Na O, K 0, PbO, ZnO, BaO, SrO, CaO, MgO, etc. when added in small amounts, i6. 3%, do not cause any major change to the characteristics of the present invention.

EXAMPLES [All percentages herein are by weight] Coefiicient of thermal expansion of crystalline glass 10- em./cm. C. (100-300 C.) 3 -5 4 3 5 11 12 14 Coefiicient of thermal expansion of glass, 10 cmJom. C. (100-300" C.) 44 44 44 32 35 33 Transformation point of glass, C 713 698 707 689 714 697 704 734 surrendering point of glass, C 778 769 784 767 777 772 777 780 Thermal treatment temperature, C 850 850 850 850 850 900 900 900 Time for thermal treatmerit, hours 2 2 2 2 2 2 2 2 The above described glass compositions were melted at a temperature of 15001630 C. for about 12 hours and the glasses thus obtained were heated at a temperature of 800-950 C. for about two hours to obtain the transparent crystalline glasses of this invention.

4 I claim: 1. A transparent crystalline glass having a low coefiicient of thermal expansion and the following range of composition by weight:

Percent Si0 -75.0 A1 0 12 0-27.0 Li O 1.7-4.5 Zr0 2.0-5.0 (SiO +Li O+Al O 2. A transparent crystalline glass having a low coefficient of thermal expansion and the following range of composition by weight:

Percent SiO 67.0-72.0 A1 0 15.0-22.0 Li O 2.2-4.5 ZrO 2.0-3.0 (SiO +Li O+Al O 92 No references cited.

TOBIAS E. LEVOW, Primary Examiner.

H. MCCARTHY, Assistant Examiner. 

1. A TRANSPARENT CRYSTALLINE GLASS HAVING A LOW COEFFICIENT OF THERMAL EXPANSION AND THE FOLLOWING RANGE OF COMPOSITION BY WEIGHT: 